Water-absorbing resins have been used in various absorbing articles. Such article include diapers, sanitary goods, soil water-retaining agent, freshness-maintaining agents, dew condensation preventing agents, sealing materials, etc.
Various water-absorbing resins and processes for producing them have been proposed. Examples of the known water-absorbing resins include a hydrolyzed product of a starch-acrylonitrile graft copolymer (as described in JP-B-49-43395), a partially neutralized material of a starch-acrylic acid graft copolymer (as described in JP-B-53-46199), a saponified material of a acrylic acid ester-vinyl acetate copolymer (as described in JP-B-53-13495), a crosslinked material of a partially neutralized acrylic acid (as described in JP-B-58-35605), a modified material of a crosslinked polyvinyl alcohol (as described in JP-A-54-20093), etc. (The term "JP-B" as used herein means an examined Japanese patent publication, and the term "JP-A" as used herein means an unexamined published Japanese patent application.)
The above-described conventional water-absorbing resins contain a certain amount of water-soluble content. The presence of the water-soluble content sometimes is not desirable in the aspect of both the performance and the safety of the water-absorbing resin. For example, when the water-absorbing resin is in contact with a liquid to be absorbed to form a hydrogel structure whereby the water-soluble content is extracted into the liquid to be absorbed, not only the absorbing properties of the resin are reduced in proportion to the extracted water-soluble content, but also deterioration of the water-absorbing resin is accelerated. When a large amount of the water-soluble content is in contact with a human body, etc., such a water-soluble content sometimes is not desirable from the viewpoint of safety as it gives unpleasant slimy feeling, etc.
Processes for producing water-absorbing resins having a small amount of water-soluble content have been proposed. Examples of such processes include a process comprising polymerizing a monomer containing both a polymerizable unsaturated group and a free acid group, followed by neutralization (as described in JP-A-62-54751), a process comprising polymerizing a low-neutralized monomer, followed by neutralization (as described in JP-A-1-144404), a process comprising irradiating a water-absorbing resin with an ultraviolet ray in the presence of a radical scavenger (as described in JP-A-4-120112), a process comprising adding a reducing substance and a radical scavenger to a water-absorbing resin (as described in JP-A-4-120111), etc.
The above-described processes each includes a new unit operation which is required in production steps. For example, in the processes of JP-A-62-54751 and JP-A-4-144404, a neutralization step is required after the polymerization. In the processes of JP-A-4-120112 and JP-A-4-120111, a step for mixing an additive with the water-absorbing resin and a step for irradiating the water-absorbing resin with an ultraviolet ray are required. In the step for neutralization of the water-absorbing resin after polymerization, and the step for mixing additives with the water-absorbing resin after polymerization, a uniform neutralization or mixing is difficult and may cause a decrease in the productivity.
It is generally known that the amount of water-soluble content can be reduced by increasing the amount of a crosslinking agent used. However, as a result of the use of an increased amount of a crosslinking agent, the absorbing properties of the water-absorbing resin decreases.
Water-absorbing resins having a small amount of water-soluble content have been known. For example, in a process for producing a water-absorbing resin by adiabatic polymerization (as described in JP-B-1-31531), a polymer gel is treated with water and a methanol solution. In this production process, a water/methanol treatment is required and therefore the productivity decreases. Further, it is probable that methanol remains in the resulting resin and, hence, the process is not desirable from the standpoint of safety.
Water-absorbing resins sometimes required to have salt resistance. For example, the absorbing properties change depending on the kind of solutions to be absorbed and depending on the lapse of time.
The water-absorbing resin containing an electrolytic structure such as the partially neutralized acrylic acid described above generally has poor salt resistance. If such a water-absorbing resin is used as a diaper, the absorbing properties are changed due to the change in concentration of salts from electrolytes in urine and the lapse of time, resulting in fluctuation of product quality.
It is known that nonionic water-absorbing resins and sulfonic group-containing water-absorbing resins are excellent in absorbing properties to electrolytic solutions. Examples thereof include the water-absorbing resin contained in the water-swellable waterproofing material (as described in JP-A-62-259846), the water-absorbing resin contained in the waterproofing material for cables (as described in JP-A-4-363383), and the waterproofing material for optical and electric cables (as described in JP-B-5-4764). However, these water-absorbing resins have low gel strength, resulting in deteriorated absorbing properties under pressure.
Water-absorbing resins having high absorbing properties under pressure have been known. For example, a water-absorbing resin having high absorbing properties under pressure is used in the absorbing article containing hydrogel having high absorption capability under pressure (as described in U.S. Pat. No. 5,147,343).
Processes for producing a water-absorbing resin having high absorbing properties under pressure also have been known. Examples thereof include a process for producing powdery polymer with selecting surfactant (as described in French Patent 8,611,742), a process for producing a water-absorbing resin by a high-concentration polymerization (as described in JP-A-63-275607), a process for producing a water-absorbing resin by forced heating (as described in JP-A-63-275608), and a process for producing water-absorbing resin by mixing a second crosslinking agent, followed by subjecting heating treatment (as described in JP-A-6-184320). However, these water-absorbing resins having high absorbing properties under pressure do not have high salt resistance.
The present invention is directed to solution of the above-described problems.